1. Field of the Invention
This invention relates to photodetectors. Particularly, this invention relates to digital alloy absorber layers for infrared photodetectors.
2. Description of the Related Art
Photodectors are electro optical devices that respond to incident light. They have a wide range of applications, including imaging. One class of photodectors operates in the infrared light range. Such devices have applications in infrared imaging including planetary exploration, industrial quality control, monitoring pollution, firefighting, law enforcement, and medical diagnosis.
In conventional semiconductor p-n junction photodetectors, the depletion layer at the junction impedes the flow of the majority carriers across the junction, while allowing the minority carriers to flow freely. Although the depletion layer enhances detector performance in this manner, it also introduces Shockley-Read-Hall (SRH) dark currents which introduce noise. The resulting dark current limits the operating temperature. However in recent years, a newly developed class of infrared photodectors has been employing an embedded barrier layer to suppress SRH currents, as well as surface leakage currents.
The barrier infrared photodetector concept is described in U.S Patent Application Publication No. 2007/0215900, published Sep. 20, 2007, by Maimon, which is incorporated by reference herein and discloses a photo-detector comprising a photo absorbing layer comprising an n-doped semiconductor exhibiting a valence band energy level, a barrier layer, a first side of the barrier layer adjacent a first side of the photo absorbing layer, the barrier layer exhibiting a valence band energy level substantially equal to the valence band energy level of the doped semiconductor of the photo absorbing layer, and a contact area comprising a doped semiconductor, the contact area being adjacent a second side of the barrier layer opposing the first side, the barrier layer exhibiting a thickness and a conductance band gap sufficient to prevent tunneling of majority carriers from the photo absorbing layer to the contact area and block the flow of thermalized majority carriers from the photo absorbing layer to the contact area. Alternatively, a p-doped semiconductor may be utilized, and conductance band energy levels of the barrier and photo absorbing layers are equalized.
Further discussion of the operation of a barrier infrared photodetector may be found in Klipstein, “‘XBn’ Barrier Photodetectors for High Sensitivity and High Operating Temperature Infrared Sensors,” Infrared Technology and Applications XXXIV, Proc. of SPIE Vol. 6940, 69402U, 2008, which is incorporated by reference herein. The article describes a barrier photodetector is a device in which the light is absorbed in a narrow bandgap semiconductor layer whose bands remain essentially flat or accumulated at the operating bias so that all carrier depletion is excluded. In a conventional photodiode below a threshold temperature, typically 130-150K for mid-wave infrared (MWIR) devices, the dark current is due to Generation-Recombination (G-R) centers in the depletion layer. In a barrier detector, the absence of depletion in the narrow bandgap semiconductor ensures that the G-R contribution to the dark current is negligible. The dark current in the barrier detector is thus dominated by the diffusion component, both above and below the threshold temperature. Therefore, at a given temperature below the threshold temperature, a barrier detector will exhibit a lower dark current than a conventional photodiode with the same cut-off wavelength. Alternatively, for a given dark current, a barrier detector will operate at a higher temperature than a conventional photodiode, provided that this temperature is below the threshold temperature. Some device architectures are presented for barrier detectors with photon absorbing layers based on InAs1-xSbx alloys and type-II InAs/GaSb superlattices (T2SL). The thermionic and tunneling components of the dark current are analyzed and shown to be negligible for typical device parameters. An operating temperature of ˜150K may be estimated for a MWIR barrier detector with f/3 optics and a cut-off wavelength of 4.2 microns.
In addition, International Patent Publication No. WO 2005/004243, published Jan. 13, 2005, by Klipstein, describes a photodetector with a reduced G-R noise, which comprises a sequence of a p-type contact layer, a middle barrier layer and an n-type photon absorbing layer, wherein the middle barrier layer has an energy bandgap significantly greater than that of the photon absorbing layer, and there is no layer with a narrower bandgap than that in the photon-absorbing layer.
Further, International Patent Publication No. WO 2007/113821, published Oct. 11, 2007, by Klipstein, describes a photodetector with a reduced G-R noise, which comprises two n-type narrow bandgap layers surrounding a middle barrier layer having an energy bandgap at least equal to the sum of the bandgaps of said two narrow bandgap layers, wherein under flat band conditions the conduction band edge of each narrow bandgap layer lies below the conduction band edge of the barrier layer by at least the bandgap energy of the other narrow bandgap layer and wherein, when biased with an externally applied voltage, the more negatively biased narrow bandgap layer is the contact layer and the more positively biased narrow bandgap layer is the photon abosorbing layer, and wherein under external bias conditions the bands in the photon absorbing layer next to the barrier layer are flat or accumulated, and the flat part of the valence band edge in the photon absorbing layer lies below the flat part of the valence band edge of the contact layer and it also lies an energy of not more than 10kTop above the valence band edge in any part of the barrier layer, where k is the Boltzman constant and Top is the operating temperature.
International Patent Application Publication No. WO 2008/061141 by Caine et al., published May 2008 and incorporated by reference herein, describes a method of making a two-dimensional detector array (and of such an array) comprising, for each of a plurality of rows and a plurality of columns of individual detectors, forming an n-doped semiconductor photo absorbing layer, forming a barrier layer comprising one or more of AlSb, AlAsSb, AlGaAsSb, AlPSb, AlGaPSb, and HgZnTe, and forming an n-doped semiconductor contact area.
However, specific designs of barrier photodetectors with high performance can be difficult to develop. The combination of barrier layer and absorber must be carefully selected to yield optimal results. Building an nBn or XBn infrared detector requires a matching pair of absorber and barrier materials with the following properties: (1) their valence band edges (Ev) must be approximately the same to allow unimpeded hole flow, while their conduction band edges (Ec) should have a large difference to form an electron barrier, (2) they must have substantially similar lattice constants, and (3) their lattice constants should also match closely to that of a readily available semiconductor substrate material that they are grown on in order to ensure high material quality and low defect density.
The lattice matching to substrate requirement is particularly important for the absorber material, because of the need to grow a thick absorber layer, typically a few microns, for high absorption quantum efficiency. The barrier is typically only a few hundred nanometers in thickness, and therefore can tolerate only a small amount of lattice-mismatching to the substrate material without suffering degradation in the barrier material quality. Because of this set of stringent requirements, initially the nBn detector was implemented with success only in a single material system, using an InAsSb infrared absorber that is lattice-matched to GaSb substrate, and an AlSbAs barrier with a matching valence band edge. The approximate composition of the lattice matched InAsSb ternary alloy is InAs0.91Sb0.09. The limitation of this implementation is that the absorber material, and hence the infrared detector made from it, has a fixed cutoff wavelength, at approximately 4.1 μm when measured at 150K.
Particular applications may require an infrared detector having a specific cutoff wavelength. In addition, it is especially desirable to have photodetectors with a cutoff wavelength of at least 5 μm for applications operating within the known atmospheric transmission window of 3 to 5 μm wavelengths.
Some newer nBn photodetectors designs employ an bi-layer superlattice absorber comprising two different semiconductor materials (e.g. GaSb and InAs) which are both lattice matched to the substrate and employed with substantially equal thickness, having substantially similar lattice constants (and therefore lattice matched to one another as well). The InAs/GaSb type-II superlattice is a well-established infrared material, with absorption cutoff wavelength that can be customized. Cutoff wavelengths ranging from below 2 μm to over 30 μm have been demonstrated. For example, Rodriguez et al., “nBn structure based on InAs/GaSb type-II strained layer superlattices,” Appl. Phys. Letters, 91, 043514, 2007, disclose a type-II InAs/GaSb strained layer superlattice photodetector using an nBn design that can be used to eliminate both Shockley-Read-Hall generation currents and surface recombination currents, leading to a higher operating temperature. The strained layer superlattice photodetector based structure is presented with a cutoff wavelength of 5.2 μM at room temperature. Processed devices exhibited a quantum efficiency around 18%, and a shot-noise-limited specific detectivity ˜109 Jones at 4.5 μm and 300 K, which are comparable to the state of the art values reported for p-i-n photodiodes based on strained layer superlattices.
In view of the foregoing, there is a need in the art for apparatuses and methods for improved barrier infrared detectors that operate with short- to mid-wavelength infrared light, e.g. approximately 1 to 5 μm. There is particularly a need for such apparatuses and methods to operate up to 5 μm wavelength in order to match the atmospheric transmission window of 3 to 5 μm. There is a need for such apparatuses and methods to operate with improved carrier transport properties. There is further a need for such apparatuses and methods to operate having a tailorable cutoff wavelength. These and other needs are met by embodiments of the present invention as detailed hereafter.